In the last ten years, numerous studies have demonstrated the major role played by melatonin (5-methoxy-N-acetyltryptamine) in numerous physiopathological phenomena and also in the control of circadian rhythm. Its half-life is, however, quite short owing to its being rapidly metabolised. It is thus very useful to be able to provide the clinician with melatonin analogues that are metabolically more stable and that have an agonist or antagonist character on the basis of which a therapeutic effect that is superior to that of the hormone itself may be expected. In addition to their beneficial action on disorders of circadian rhythm (J. Neurosurg. 1985, 63, pp 321-341) and sleep disorders (Psychopharmacology, 1990, 100, pp 222-226), ligands of the melatoninergic system have valuable pharmacological properties in respect of the central nervous system, especially anxiolytic and antipsychotic properties (Neuropharmacology of Pineal Secretions, 1990, 8 (3-4), pp 264-272) and analgesic properties (Pharmacopsychiat., 1987, 20, pp 222-223), and also for the treatment of Parkinson's disease (J. Neurosurg. 1985, 63, pp 321-341) and Alzheimer's disease (Brain Research, 1990, 528, pp 170-174). Those compounds have also shown activity on certain cancers (Melatonin—Clinical Perspectives, Oxford University Press, 1988, pp 164-165), ovulation (Science 1987, 227, pp 714-720), diabetes (Clinical Endocrinology, 1986, 24, pp 359-364), and in the treatment of obesity (International Journal of Eating Disorders, 1996, 20 (4), pp 443-446).
Those various effects take place via the intermediary of specific melatonin receptors. Molecular biology studies have shown the existence of a number of receptor sub-types that can bind the hormone (Trends Pharmacol. Sci., 1995, 16, p 50; WO 97.04094). It has been possible to locate some of those receptors and to characterise them for different species, including mammals. In order to be able to understand the physiological functions of those receptors better, it is very valuable to have specific ligands available. Moreover, by interacting selectively with one or other of those receptors, such compounds can be excellent medicaments for the clinician in the treatment of pathologies associated with the melatoninergic system, some of which have been mentioned above.
In addition to the fact that the compounds of the present invention are new, they exhibit very great affinity for melatonin receptors and/or selectivity for one or other of the melatoninergic receptor sub-types.